First Year Organic Chemistry
THE CHEMISTRY OF THE CARBONYL GROUP:
CORE CARBONYL CHEMISTRY
Professor Tim Donohoe
8 lectures, HT, weeks 1-4, 2017
Weeks 1 +3 Monday at 10am; Wednesday at 9am (Dyson Perrins)
Weeks 2 +4 Wednesday at 9am; Thursday at 10am (Dyson Perrins)
Handout A
You will be able to download copies of the handouts from this course at http://donohoe.chem.ox.ac.uk/Teaching/Teaching.htm as well as through Weblearn
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Course Structure 1) Nucleophilic addition to C=O A) Nucleophiles and electrophiles: General principles B) Reversible addition (hydrates and hemiacetals) C) Irreversible addition (organometallic addition and reduction)
2) Nucleophilic substitution of C=O A) Acetals B) Imines, oximes and hydrazones C) Formation of C=C bonds from carbonyls D) Removal of C=O from carbonyls
3) Nucleophilic substitution at C=O A) Tetrahedral intermediates in substitution; B) Factors that affect reactivity of C=O towards nucleophiles; leaving group ability; IR spectroscopy C) The reactivity of acid chlorides (RCOCl)
D) The reactivity of anhydrides (RCO)2O E) The reactivity of esters COOR
F) The reactivity of amides CONR2
4) Enolisation of carbonyl compounds A) keto-enol tautomerism B) enols and enolates as nucleophiles C) condensation reactions with carbonyl groups D) conjugate additions
Suggested Reading: Core Carbonyl Chemistry, J. Jones, Oxford Primer Organic Chemistry, Clayden, Greeves, Warren and Wothers Organic Chemistry, Volhard and Schore A guidebook to mechanism in organic chemistry, Sykes The Chemistry of the Carbonyl Group, Warren
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1. Nucleophilic addition to C=O A) Nucleophiles and Electrophiles
Structure of carbonyls consider the and framework
MO picture of a C=O
Antibonding orbital resembles
A p-orbital on carbon
a p-orbital on O
Bonding orbital resembles
So, C=O have a low energy (unfilled) * orbital that has a large coefficient on carbon and this is crucial to its reactivity.
Canonicals show the C is electron deficient
In order to break a bond we place two electrons in the antibonding orbital; the bond order then becomes
Bond order is:
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When nucleophiles attack the C=O group they do so by passing electrons from their highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the carbonyl ie. Negatively charged species are also attracted to the electron deficient carbon atom.
So, in the addition of cyanide to acetone, the following electron movements are involved. a) Curly arrow representation
b) orbitals involved
All additions to C=O follow the same pattern of events, but the nature of the HOMO depends on the particular nucleophile used. Once you understand the orbitals involved you do not need to draw the orbitals for every addition to a carbonyl.
We must make a distinction between reversible and irreversible additions:
B Reversible addition: eg. The addition of cyanide can be reversed by adding a base
- This happens because CN is a good
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The addition of water is also reversible and observed through the formation and collapse of hydrates
O O HO OH
H2O
hydrate of ketone
For this reversible reaction, the thermodynamic stability of the carbonyl versus the hydrate will determine the percentage of hydrate at equilibrium.
Standard ketones (acetone) contain very little hydrate:
Keq (in water, 25°C) Keq (in water, 25°C)
O O 18 36 H H Cl3C H
O O 0.01 22000 Me H F3C CF3
O O 1.8 x10-5 Me Me
Factors influencing extent of hydration i) Steric hindrance: repulsion between groups that are close in space:
ii) Electron withdrawing groups. Inductive effect increases the reactivity of the C=O to nucleophiles
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iii) Delocalisation (conjugation)
O O O O
Me Me Me Me
These three factors influence other C=O reactions too. Of course, the addition of alcohols to C=O is also easy (and reversible).
O MeO OH MeOH + Me H Me H
Some hemiacetals are stable because the alcohol attacks in an
OH O O OH O O
The formation of hemiacetals is catalysed by either ACID or BASE In ACID
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In BASE
Further reading: look up the (reversible) addition of bisulfite to carbonyl compounds and also the Meerwein Pondorff Verley reduction.
C. Irreversible addition at a carbonyl is perhaps more common i) Organolithium reagents are very reactive:
ii) Addition of organomagnesium reagents, such as Grignards, is v. important in synthesis
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formation of Grignard reagents
I Mg ether
Br
These organometallic reagents add to C=O, although the precise details of the attack are complex because the metal ion acts as a Lewis acid.
O Mg H+ R R
O H+ R H
Mg
Reduction of carbonyl compounds is observed when bulky Grignards are used e.g. tBuMgBr:
Mg O O
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We see a similar pattern of reactivity during the Cannizzaro reaction:
(i) NaOH (conc.) 2 PhCHO + (ii) H3O
The mechanism involves base catalysed addition of hydroxide to the aldehyde; followed by hydride transfer.
Q. Why does this reaction only work with aldehydes that have NO alpha protons?
However, reduction of a carbonyl is best accomplished with NaBH4 or LiAlH4 Ketones are reduced to
Aldehydes are reduced to
Reaction mechanism with LiAlH4 is more complex and takes place in an inert solvent such as ether (this is because
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O + O + Li LiAlH4 H
R H R H Et2O
Al
2. Nucleophilic substitution of C=O
A) Acetals: In acid, hemiacetal formation from an aldehyde or ketone does not The acid allows
O
R H R H R H
R H R H R H
The product is an
Remember, acetals only form in
Also
This process is an equilibrium and can be shifted in either direction by removal of the products or addition of excess of one reagent.
To form an acetal use:
To hydrolyse an acetal use:
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Acetals are stable to base, nucleophiles and oxidants; so they are commonly used as
O
FG FG FG FG
B) Formation of Imines and related derivatives from carbonyls
Nitrogen based nucleophiles also add to carbonyl compounds: consider attack of a primary amine at a ketone.
CH3-NH2 O
Other amine derivatives add to carbonyl compounds in an analogous manner.
HO–NH2
H2N–NH2
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These condensations are very pH dependent
HONH2 O +
Step 1
Step 2
rate
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Aside on 2° amines: Note that secondary amines cannot condense with a carbonyl to produce a neutral compound
O N H potent electrophile: H H H H see the
N H
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And, just like aldehydes and ketones, imines are useful electrophiles although they are less electrophilic (because nitrogen is less electronegative than oxygen)
A key step in the synthesis of Valsartan (Diovan)
This is called reductive amination: a method for converting aldehydes and ketone to amines Bearing in mind the reaction of aldehydes and ketones with cyanide, we can rationalise the Strecker reaction
C) Formation of C=C bonds from carbonyls i) Making alkenes from carbonyl compounds: the Wittig reaction (which consists of 1) Reaction of an alkyl halide with triphenylphosphine
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2) Treatment of the phosphonium salt with strong base to make an YLID
3) Immediate reaction of the ylid with a carbonyl compound to form an alkene
One of the best ways for making alkenes:
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Ester stabilised ylids work fine but can sometimes be unreactive. Therefore, use a more reactive nucleophile:
i) More reactive phosphorous derived compounds: the Horner Wadsworth Emmons reaction
D) Removal of C=O from carbonyls: the Wolff Kishner reaction It is sometimes useful to be able to remove a C=O completely from a molecule. There are several ways of doing this, dependent upon whether the molecule can tolerate acid or base.
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3. Nucleophilic substitution at C=O A) Tetrahedral intermediates in substitution
Overall, the substitution process can be represented as: O O Nu
R X R Nu
This reaction does NOT go through a direct displacement: instead, the nucleophile finds it easier to add to the carbonyl group (the * is lower in energy and more accessible to the HOMO of the nucleophile than a * orbital).
The intermediate (known as a TETRAHEDRAL INTERMEDIATE) can do two things,
O Nu R X R X R Nu OR
Lets focus on each step of this mechanism.
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B) Step 1: How does the nature of X affect the reactivity of the carbonyl group towards nucleophiles?
There are two effects here:
(i) Inductive electron withdrawal
O O O
R R R
Increased electronegativity of X
(ii) Conjugation of a lone pair on X with the C=O
Think about the shape of the ester oxygen
O
R O
In molecular orbital terms:
By conjugating the two species
the LUMO
and the HOMO
R R O O OR O
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B) Step 2: Leaving group ability determines which product is formed
Leaving group ability: correlation with pKa How do we know which is the best leaving group?
X
There is already a scale that can help us: pKa: H X
Large values of pKa mean small values of Ka ie
Small values of pKa mean large values of Ka ie
Leaving group X– pKa of H-X Me H
NH2 EtO HO
MeCO2 Cl
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Probing the nature of the carbonyl group by Infra-red (IR) spectroscopy
IR spectroscopy measures
Can be described using Hooke’s Law:
Remember that
So, strong bonds absorb at high
The factors discussed earlier will influence the strength of the C=O bond in the following ways:
1) Delocalisation WEAKENS
2) Inductive effects STRENGTHEN
The derivatives shown earlier have a combination of the 2 effects and this can be seen in the IR. Compare the C=O stretch of
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Functional groups in action. C) X= chlorine then we have an acid chloride which are very reactive species because
O ROH R Cl R Cl R Cl R
N H R Cl R Cl R Et3N
Note that a base must be present here because
You can make acid chlorides from carboxylic acids like this:
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D) When X=OCOR these are called anhydrides and are slightly less reactive than acid chlorides
O O So, oxygen shares its
R O
As one would expect, reaction of anhydrides mirrors that of acid chlorides
O O R2NH ROH O
E) X= OR, esters Esters are substantially less reactive towards nucleophiles than aldehydes and ketones;
O O NaBH4, MeOH
O
Esters do react, but only with more powerful nucleophiles, eg NaOH
O Na O H Me Me R O R O R O
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We can also increase the reactivity of esters by using ACID catalysis
Drive reaction to completion by using an excess of water or remove the alcohol by-product
Further reading: the acid and base catalysed hydrolysis of esters can be classified into 8 different catagories (AAC1, AAC2, AAL1, AAL2, BAC1, BAC2, BAL1, BAL2) depending upon the mechanism-see J. March, Advanced Organic Chemistry, Fourth Ed, P378. Given the above, the following should come as no surprise: 1) Reaction with an amine ()
2) Reduction with LiAlH4
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So, what happens if we try to make a ketone via reaction of an ester with
O Me Me O O Me
In fact, this is a good method for making tertiary alcohols whereby two R groups are the same
Clearly there is a problem in making ketones with this chemistry. Three solutions are available. 1) React a carboxylic acid with TWO equivalents of a reactive organolithium reagent
H O O
MeLi
2) Use an acid chloride rather than an ester; AND decrease the reactivity of the nucleophile by changing the metal counterion from lithium to
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Solution 3 can wait until we have discussed amides:
F) X= NR2, amides These are the least reactive of the derivatives (towards nucleophiles) discussed so far because
As the constituents of poly amides (ie peptides) these functional groups are essential parts of biological systems. We can hydrolyse an amide bond in the laboratory, but require harsh acidic or basic conditions to do it
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Generally, acid is better than base for hydrolysing amide, although strong bases such as can do the hydrolysis.
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Think about the reduction of amides with LiAlH4
A simple way of making substituted amines involves coupling of an acid chloride with an amine to give an amide, followed by
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Now we can return to solution 3 for making ketones from addition to carbonyl compounds without over-reaction. 3) Use a Weinreb’s amide
The chelate is Stable
Doesn’t
Quenching with acid destroys the
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The following scheme says it all Increased reactivity: Increased leaving group ability
O -7 Cl
O O 5 O
O O 16 OH OR
O 30 NR2
Increasing pKa of leaving group's conjugate acid
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Finally, note the central position that carboxylic acids have- they can be transformed into
Recall methods for making carboxylic acids:
The pKa of a carboxylic acid can tell us a lot about the nature of the Advanced reading: for a comprehensive list of pKa values for organic compounds (and more) see: http://research.chem.psu.edu/brpgroup/pKa_compilation.pdf
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